Heretofore, subsea wellheads have been provided in which an outer tubular wellhead housing is secured to the sea floor and an inner tubular wellhead housing is supported within the outer tubular wellhead housing. The inner wellhead housing has a hub projecting upwardly above the outer wellhead housing. A hydraulic wellhead connector forms a robust and disconnectable structural link from the drilling vessel's riser extending to the subsea wellhead below. A connector which may be connected at its upper end to a blowout preventer stack is latched on the hub of the wellhead inner housing. Thus, any external tension or bending loads are first transferred from the connector to the inner wellhead housing, and then transferred from the inner wellhead housing to the outer wellhead housing.
Such a wellhead structure is shown in U.S. Pat. No. 5,066,048 dated Nov. 19, 1991 in which the hub of an inner tubular wellhead housing extends upwardly above the outer tubular wellhead housing and has resilient fingers engaging the outer housing to effect preloading between the inner and outer wellhead housings. External tension and bending loads imparted at the connector to the inner wellhead housing interface are transferred to the outer wellhead housing through the inner housing.
The connector which connects to the hub of the inner housing is utilized in subsea drilling operations to connect a marine riser and blowout preventer stack to the wellhead. Subsea wells are being drilled at increased water depths over four thousand feet and some as deep as eight thousand feet. Particularly for "guidelineless" drilling from a drill ship or floating drilling platform, the drill ship or platform may be off location a substantial distance resulting from station keeping failures, tides, wind, water currents and the like. When the drill ship is off location, bending loads are generated through the interconnecting marine riser into the wellhead. Thus, the subsea wellhead and connector must be capable of withstanding the extreme bending loads resulting from the increased water depths at which this drilling is occurring. The connector provides a connection between the blowout preventer stack and the marine wellhead, and also affects a metal-to-metal seal at the wellhead. The connector should be designed to withstand any reasonable bending and separation forces caused by well pressure, riser tension, and bending loads created by motion from the drill ship or platform above.
U.S. Pat. No. 4,693,497 dated Sep. 15, 1987 shows a collet connector for connecting two axially aligned tubular members including latching fingers which engage outer shoulders on axially aligned tubular members for connecting the tubular members. When the collet connector is utilized for connecting a marine riser and blowout preventer stack to a subsea wellhead, the outer tubular member secured to the sea floor may, for example, be thirty six (36) inches or thirty (30) inches, in diameter while the inner tubular housing is about eighteen and three-fourths (183/4) inches in diameter. All external bending and tension loads from the riser and blowout preventer (BOP) stack are transferred through the connector to the inner tubular housing through the interface between the inner tubular wellhead housing and the outer tubular wellhead housing, and eventually into the earth. The inner tubular housing also must be designed to withstand potentially high internal fluid pressure and the attending pressure end load imposed on the connector. The combined internal pressure and very high external loading resulting from deep water wells, which may be in water depths over five thousand (5,000) feet, may be as high as eight (8) million foot pounds.
It is an object of the present invention to provide a subsea wellhead system for deepwater drilling operations in which a subsea wellhead secured to the sea floor is capable of safely reacting large marine riser induced bending and tension loads.
It is a further object of this invention to provide such a wellhead system in which a connector for connecting the marine riser and blowout preventer stack to the wellhead is connected directly to the outer wellhead housing which is secured to the sea floor for transferring external tension and bending loads from the connector directly to the outer wellhead housing thereby bypassing the inner wellhead housing.